Flames from the Mountain Fire have destroyed 23 structures, including seven homes the first day of the fire.

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A female inmate hand crew from and firefighters in an engine company with them set fire to reinforce the line to stave off part of the Mountain Fire burning up a hill toward them on Tuesday, July 16, 2013.

Updated at 7:19 PM PDT on Thursday, Jul 25, 2013

The massive Mountain Fire that has destroyed homes and is still burning in the San Jacinto Mountains was sparked on private property by failed electrical equipment, authorities revealed Thursday, 10 days after the wildfire erupted.

"The electrical equipment failure occurred on the customer side of the meter," Cal Fire said in a news release. "No further details will be released as this remains an on-going investigation.”

The Mountain Fire was reported in the early afternoon July 15 near the junction of Highway 324 and Highway 74.

In 10 days, the wildfire had scorched 42 square miles in the scenic San Jacinto mountain range. It was 92 percent contained Thursday evening, according to an online incident report from Cal Fire.